The present disclosure relates to a light-source assembly used in conjunction with tracking, angle-measuring devices to determine coordinates of points or surfaces.
An instrument commonly referred to as a laser tracker has the ability to measure the coordinates of a point by sending a laser beam to a retroreflector target that is in contact with the point. A laser tracker is often used in conjunction with a particular type of retroreflector target called the spherically mounted retroreflector (SMR). The SMR comprises a cube-corner mounted within a sphere with the vertex of the cube-corner at the sphere center. A gimbal mechanism within the laser tracker directs a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker uses the position of the light on the position detector to adjust the rotation angle of mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the laser beam is able to track an SMR that is moved over the surface of an object of interest. Part of the light retroreflected into the laser tracker passes onto a distance-measuring device such as an interferometer or absolute distance meter (ADM). Angular encoders attached to the mechanical azimuth and zenith axes of the tracker measure the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance and two angles measured by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
The cube-corner retroreflector embedded within the sphere in the SMR comprises three mutually perpendicular reflecting surfaces. These three reflecting surfaces occupy three of the six faces of a cube. The diagonal of this cube is the axis of symmetry of the cube corner and, hence, also the axis of symmetry of the SMR. To avoid problems caused by optical effects such as vignetting, the axis of symmetry of the SMR must ordinarily be aligned to within about 40 degrees of the incident laser beam. Because of this limitation, it is difficult to make certain types of measurements with an SMR. For example, it is difficult to measure the coordinates of an SMR attached to a machine tool that is moved over a large distance or rotated over a large angle. As another example, it is sometimes desirable to simultaneously measure a target with several widely separated laser trackers to improve measurement accuracy. However, the SMR is impractical for such measurements because of its limited angle of acceptance. Furthermore, in most applications, the SMR is carried by hand over the surfaces of interest. If the SMR is tilted too far from the laser beam, the tracker measurement will be interrupted until the SMR is rotated back into position. If an interferometer is being used to measure the distance, then it will be necessary to move the SMR back to a reference position before resuming the measurement. Such interruptions can be annoying and time consuming.
In most laser trackers today, the accuracy of the distance and angle measurements is approximately proportional to the distance of the SMR from the laser tracker, with the constant of proportionality given in units of micrometers/meter or, equivalently, in parts per million (ppm). State-of-the-art laser trackers in a typical environment have a radial accuracy, determined by the accuracy of the interferometer or ADM, of approximately 2 ppm. Today, these same trackers have an angular accuracy, determined by a variety of factors including the accuracy of the angular encoders, of approximately 10 ppm. In other words, the main source of error in most measurements is angular rather than radial. The angular accuracy of the laser tracker could be improved to approach the radial accuracy by using larger and more expensive angular encoders together with air bearings, and a stiffer, a thermal mechanical structure. However, a laser tracker modified in this way would be noisier, less portable, and much more expensive than trackers available today. Rather than increase the cost of the laser tracker, it would be preferable to reduce cost while maintaining or improving measurement accuracy.
To measure three-dimensional coordinates, an alternative to the retroreflector target is a point source of light formed by sending light out of an optical fiber and through a dispersing element. Schultz et al. describe this type of target in patent number U.S. Pat. No. 5,907,395.